Bonds Of Cyclohexane Research Articles

Spectroscopic Characterization of the 1-Boratricyclo-[4.1.0.02,7]-heptane Radical with a Delocalized Four-Center-One-Electron Bond.

The boron atom is a highly electrophilic reagent due to the presence of its empty p orbital, making it prone to undergo electrophilic addition reactions with the carbon-carbon double bonds of olefins. In this study, the classical C=C reaction pathway occurs when a boron atom attacks the C=C bond of cyclohexene, resulting in the formation of the η2 (1,2)-BC6H10 complex (A) that contains a borirane radical subunit. This complex can further undergo photoisomerization, leading to the formation of a 3,4,5,6-tetrahydroborepine radical (C) through the cleavage of C-C bonds. In addition, two 1-boratricyclo[4.1.0.02,7]heptane radicals with chair (B) and boat (B') conformations were observed through α C-H cleavage reactions. Bonding analysis indicates that these radicals involve a four-center-one-electron (4c-1e) bond. Under UV light irradiation, these two radicals undergo ring-opening and rearrangement reactions, resulting in the formation of a 1-cyclohexen-1-yl-borane radical (D), which is a sp2 C-H activation product. These findings delineate a potential pathway for the synthesis of organoboron radicals through boron-mediated C-H and C-C bond cleavage reactions in cycloolefins.

MnxWO4 Nanostructure-Based Catalysts for Single-Step Oxidation of Cyclohexane and Methane to Oxygenates

Activation of the C–H bond in cyclohexane (CYH) and methane is a crucial step to obtain desirable oxygenated products using nanostructured catalyst and is a great challenge and an efficient route to mitigate the inauspicious effects of climate change. The active sites were identified using XRD, HR-TEM, SEM, N2 sorption analysis, TPR, Raman, XPS, TGA, in situ DRIFT, XAS, etc. In optimal reaction conditions, 46% of CYH was converted into adipic acid (AA) on MnxWO4 nanostructures within 6 h. The recyclability test confirmed the catalyst heterogeneity, which revealed no appreciable loss of catalytic activity even after three consecutive reactions. In situ DRIFT study reveals that CYH is oxidized to cyclohexanone and cyclohexanol (KA oil) and is further oxidized to AA via carboxylate intermediates. DFT studies disclosed that MnOx species are responsible for the C–H activation of CYH, and the Mn2+/Mn3+ redox centers play a vital role in the absorption of KA oil to form AA. Herein, we demonstrated the significant role of the “MnOx” species and that adequate Lewis and Bronsted acidic sites, redox centers of (Mn2+/Mn3+), and lattice oxygen are accountable for the CYH conversion toward the AA. Additionally, we have reported the oxidation of methane to methanol (146 μmole per gram of catalyst) in the presence of water at 75 °C without over-oxidation products.

Anionic Polymerization of 4-Allyldimethylsilylstyrene: Versatile Polymer Scaffolds for Post-Polymerization Modification

A facile synthesis using a Grignard reaction was employed to prepare the silicon containing functional monomer 4-allyldimethylsilylstyrene (4ADSS). Detailed studies regarding the living nature of anionic polymerization of 4ADSS and its polymerization via the styrene vinyl bond in cyclohexane at room temperature were conducted, and P4ADSS samples with Mn up to 80 kg mol–1 were accessible. This evidences that the 4ADSS structure disables undesired proton transfer side reactions, as known for the carbon-based analogue functional monomer 4-but-3-enyl-styrene. Real-time 1H NMR kinetics of the statistical copolymerization of 4ADSS with styrene (r4ADSS = 3.55; rS = 0.047) revealed a remarkable gradient microstructure in the resulting copolymers. Based on this observation, a library of well-defined gradient copolymers P(4ADSS-co-S) with Mn in the range of 5–50 kg mol–1 was synthesized, varying 4ADSS content. A comprehensive discussion regarding the glass transition temperatures (Tg) of the synthesized homo- and copolymers is presented. Furthermore, thiol–ene click reactions at the pendant allyl moiety of P(4ADSS-co-S) were explored to introduce functional groups. Likewise, P(4ADSS-co-S) copolymers were subjected to hydrosilylation reactions, and the impact of the introduced siloxane moieties on the glass transition of the resulting copolymers is presented. Both thiol–ene reactions as well as hydrosilylation of the P(4ADSS-co-S) copolymers proceeded quantitatively. Consequently, copolymerization of 4ADSS provides access to a wide range of polystyrene-based functional materials with specific applications using versatile post-polymerization chemistry.

Photochemical reactions of a diamidocarbene: cyclopropanation of bromonaphthalene, addition to pyridine, and activation of sp3 C-H bonds.

We report unprecedented photochemistry for the diamidocarbene 1. Described within are the double cyclopropanation of 1-bromonaphthalene, the double addition to pyridine, and remarkably, the insertion into the unactivated sp3 C-H bonds of cyclohexane, tetramethylsilane, and n-pentane to give compounds 2-6, respectively. All compounds have been fully characterized, and the solid state structure of 4 was obtained using single crystal electron diffraction.

Biosynthesis of Gold Clusters and Nanoparticles by Using Extracts of Mexican Plants and Evaluation of Their Catalytic Activity in Oxidation Reactions

This work shows the biosynthesis of gold clusters and nanoparticles (AuNPs) from a solution of gold in a mixture of ammonium chloride and nitric acid treated with two different Mexican plants extracts, Toad Leaf (Eryngium heterophyllum, SF) and Cuachalalate (Amphipterygium adstringens, CF) as bioreductants. Subsequently, the gold entities were supported on the ordered mesoporous material SBA-15 functionalized with mercaptopropyl groups (1.6 wt% of sulphur). The gold contents were 0.7 wt% and 1.1 wt% for the CF and SF samples, respectively. UV–vis spectroscopy and TEM show the presence of gold nanoclusters in both samples, while very minor amount of AuNPs is detected only in the sample with the highest Au content (SF). The Au/SBA-15 materials are active in the aerobic oxidation of cyclohexene at 65 °C and atmospheric pressure. Two of the catalytic reaction products come from the addition of oxygen to the double bond of cyclohexene: cyclohexene epoxide and cyclohexanediol, while the other three come from allylic oxidation of the ring, which is the major reaction pathway: 2-cyclohexen-1-ol, 2-cyclohexen-1-one and 2-cyclohexenyl hydroperoxide, being the two first the major stable reaction products, while the enone/enol ratio is nearly 2. It has been found that the gold nanoclusters initially present in the catalysts evolve toward AuNPs during catalytic reaction.

Cooperative effect of hetero-nuclear MnNi+ cation enhancing C–H bond activation of cyclohexane: a theoretical study

Hetero-nuclear MnNi+ cation exhibits high catalytic activity in the C–H bond activation of cyclohexane in comparison with mono-nuclear Mn+ cation. The dehydrogenation reaction mechanistic investigations of cyclohexane catalyzed by MnNi+ and Mn+ have been carried out with density functional theory calculations. The activations of the first, third and fifth C–H bond of cyclohexane occur mainly because of the orbital interaction between the C–H σ bond of cyclic hydrocarbon and the empty d orbital of Mn. The smaller energy gaps of interacting orbitals result in stronger electron transfer between the related orbitals, which can elucidate MnNi+ cation is more reactive than Mn+. The coordination bonds between MnNi+ and cycloolefin in the second and third dehydrogenation processes of cyclohexane are another important factor for accelerating the dehydrogenation, which can stabilize the transition states for the activation of C–H bonds. The lower the transition state energy, the easier the C–H bond activation.

Сравнительное DFT исследование триплетных и синглетных элементарных актов окисления циклогексана и 1,3-циклогексадиена, инициированных первичным взаимодействием с 3O2 в СКФ-условиях

The primary stages of the oxidation of model cyclohexane and 1,3-cyclohexadiene by triplet molecular oxygen and subsequent transformations involving triplet and singlet states were studied for the first time by the DFT method with the density functional B3LYP with the basis set 6-311++g(df,p). It was shown that, ceteris paribus, cyclohexane and 1,3-cyclohexadiene will be orders of magnitude more reactive compared to the activity of acyclic saturated hydrocarbons under SCF conditions when the oxidation process is initiated by the primary reaction with 3O2, which allows the propane-butane mixture to be effectively used as SCF conditions of heavy oils and use air purge to activate this process. The triplet associate complexes resulting from the oxidative cleavage of the secondary C–H bond of cyclohexane and 1,3-cyclohexadiene consist of hydrogen-bonded hydroperoxyl radical and cyclohexyl radical or 1,3-cyclohexadiene radical, respectively. These complexes can dissociate into unbound pairs of radicals, and therefore further reactions can proceed in the triplet or singlet direction. The singlet direction is characterized by hydrate-induced hydroperoxide-carbonyl transformation, as well as other hydrate-induced rearrangements. The triplet direction is characterized by the occurrence of triplet rearrangement, which in its essence is a triplet recombination of associated radicals. Associate triplet complexes can be agents of radical hydroperoxyl and alkyl activity, as well as agents of radical hydroxyl and alkoxyl activity. Most oxidative dehydrogenation reactions are absolutely real under a number of conditions, namely, they must take place under SCF conditions, as well as in the presence of an excess of SCF solvent necessary for the effective shift of thermodynamic equilibrium towards the target products in accordance with the Le Chatelier principle.

Regioselective Oxidation of C-H Bonds in Unactivated Alkanes by a Vanadium Superoxo Catalyst Bound to a Supramolecular Host.

A vanadyl ion bound to a cucurbituril (CB) host was reported to oxidize pentane to 2-pentanol in the presence of an oxidizer. DFT calculations suggest that the catalyst selectively reacts with stronger C-H bonds in pentane over weaker C-H bonds in cyclohexane due to size exclusion by the CB host. The active catalyst is an unprecedented vanadium superoxo species bound to the host, and the selectivity toward secondary over the primary C-H bond is the result of a higher degree of charge transfer from the secondary compared to the primary position.

Selective Alkane C–H Bond Oxidation Catalyzed by a Non-Heme Iron Complex Featuring a Robust Tetradentate Ligand

An iron complex, [FeII(BpyPY2Me)(CH3CN)2](OTf)2 (1-Fe, where BpyPY2Me is 6-(1,1-di(pyridin-2-yl)ethyl)-2,2′-bipyridine and OTf is triflate), supported by an oxidatively rugged tetradentate ligand is reported for the catalytic oxidation of unactivated C–H bonds in cyclohexane and adamantane. With the use of m-chloroperbenzoic acid (mCPBA) as the terminal oxidant, the iron catalyst shows an alcohol-to-ketone (A/K) ratio of 7.5 for cyclohexane oxidation with conversion percentages as high as 90% with respect to oxidant. Moreover, catalysis toward adamantane oxidation shows high regioselectivity (3°/2° = 45) favoring tertiary C–H bonds with yields up to 87%. Results, including electrospray ionization mass spectrometry and UV–vis spectroscopy, indicate that a molecular non-heme iron(IV)-oxo intermediate is the catalytically active species.

Relationship between Hydrogen-Atom Transfer Driving Force and Reaction Rates for an Oxomanganese(IV) Adduct.

Hydrogen atom transfer (HAT) reactions by high-valent metal-oxo intermediates are important in both biological and synthetic systems. While the HAT reactivity of FeIV-oxo adducts has been extensively investigated, studies of analogous MnIV-oxo systems are less common. There are several recent reports of MnIV-oxo complexes, supported by neutral pentadentate ligands, capable of cleaving strong C-H bonds at rates approaching those of analogous FeIV-oxo species. In this study, we provide a thorough analysis of the HAT reactivity of one of these MnIV-oxo complexes, [MnIV(O)(2pyN2Q)]2+, which is supported by an N5 ligand with equatorial pyridine and quinoline donors. This complex is able to oxidize the strong C-H bonds of cyclohexane with rates exceeding those of FeIV-oxo complexes with similar ligands. In the presence of excess oxidant (iodosobenzene), cyclohexane oxidation by [MnIV(O)(2pyN2Q)]2+ is catalytic, albeit with modest turnover numbers. Because the rate of cyclohexane oxidation by [MnIV(O)(2pyN2Q)]2+ was faster than that predicted by a previously published Bells-Evans-Polanyi correlation, we expanded the scope of this relationship by determining HAT reaction rates for substrates with bond dissociation energies spanning 20 kcal/mol. This extensive analysis showed the expected correlation between reaction rate and the strength of the substrate C-H bond, albeit with a shallow slope. The implications of this result with regard to MnIV-oxo and FeIV-oxo reactivity are discussed.

Theoretical investigation on cyclohexane dehydrogenation catalyzed by V2+ in gas-phase

The dehydrogenation reaction mechanism of cyclohexane catalyzed by dimer transition metal cluster V2+ has been investigated at the B3LYP/6-31G (d, p) level of density functional theory. Density of states (DOS) graph is used to understand more deeply the roles of the front molecular orbital of the initial complexes. After the first molecular dehydrogenation, the reaction mainly consists of two competition mechanisms. First, the C-H bonds of cyclohexane can be effectively activated by the V2+ cation, yielding the same-face dehydrogenation products. Second, the C-C bonds are activated, forming the different-face dehydrogenation products. Our calculations indicate that the reaction takes place more easily along the low-spin potential energy surface on the same-face and is a low-barrier or even barrier-free transformation. Carbon-carbon single bonds are nonpolar and generally far less reactive. A comparison of the reaction mechanism of V2+ and congener Ti2+ with cyclohexane has been presented. The bond dissociation energies (BDEs) of V2+ are greater than that of Ti2+, leading to difficulties in forming sandwich complexes in the different-face dehydrogenation of cyclohexane, and the same-face dehydrogenation is an important reaction channel.

11-trifluoromethyl-phenyldiazirinyl neurosteroid analogues: potent general anesthetics and photolabeling reagents for GABAA receptors.

While neurosteroids are well-described positive allosteric modulators of gamma-aminobutyric acid type A (GABAA) receptors, the binding sites that mediate these actions have not been definitively identified. This study was conducted to synthesize neurosteroid analogue photolabeling reagents that closely mimic the biological effects of endogenous neurosteroids and have photochemical properties that will facilitate their use as tools for identifying the binding sites for neurosteroids on GABAA receptors. Two neurosteroid analogues containing a trifluromethyl-phenyldiazirine group linked to the steroid C11 position were synthesized. These reagents, CW12 and CW14, are analogues of allopregnanolone (5α-reduced steroid) and pregnanolone (5β-reduced steroid), respectively. Both reagents were shown to have favorable photochemical properties with efficient insertion into the C-H bonds of cyclohexane. They also effectively replicated the actions of allopregnanolone and pregnanolone on GABAA receptor functions: they potentiated GABA-induced currents in Xenopus laevis oocytes transfected with α1β2γ2L subunits, modulated [(35)S]t-butylbicyclophosphorothionate binding in rat brain membranes, and were effective anesthetics in Xenopus tadpoles. Studies using [(3)H]CW12 and [(3)H]CW14 showed that these reagents covalently label GABAA receptors in both rat brain membranes and in a transformed human embryonal kidney (TSA) cells expressing either α1 and β2 subunits or β3 subunits of the GABAA receptor. Photolabeling of rat brain GABAA receptors was shown to be both concentration-dependent and stereospecific. CW12 and CW14 have the appropriate photochemical and pharmacological properties for use as photolabeling reagents to identify specific neurosteroid-binding sites on GABAA receptors.

DFT Mechanistic Proposal of the Ruthenium Porphyrin-Catalyzed Allylic Amination by Organic Azides

A DFT-based theoretical analysis describes the allylic amination of cyclohexene by 3,5(CF3)2phenylazide catalyzed by [Ru](CO) ([Ru]= Ru(TPP), TPP = dianion of tetraphenylporphyrin). The activation of an azide molecule (RN3) at the free ruthenium coordination site allows the formation of a monoimido complex [Ru](NR)(CO) with the eco-friendly dismissal of a N2 molecule. The monoimido complex can undergo a singlet→triplet interconversion to confer a diradical character to the RN ligand. Hence, the activation of the allylic C–H bond of cyclohexene (C6H10) occurs through a C–H···N interaction over the transition state. The formation of the desired allylic amine follows a “rebound” mechanism in which the nitrogen and carbon atom radicals couple to yield the organic product. The release of the allylic amine restores the initial [Ru](CO) complex and allows the catalytic cycle to resume by the activation of another azide molecule. On the singlet PES, the CO ligand may however be eliminated from the monoimido complex [Ru](NR)(CO)S, opening the way to an alternative catalytic cycle which also leads to allylic amine through comparable key steps. A second azide molecule occupies the vacant coordination site of [Ru](NR)S to form the bis-imido complex Ru(TPP)(NR)2, which is also prone to the intersystem crossing with the consequent C–H radical activation. The process continues until the azide reactant is present. The interconnected cycles have similarly high exergonic balances. Important electronic aspects are highlighted, also concerning the formation of experimentally observed byproducts.

Oxidation of Toluene and Other Examples of CH Bond Activation by CdO2 and ZnO2 Nanoparticles.

Nanoparticles of CdO2 and ZnO2 are shown to oxidize toluene primarily to benzaldehyde in the 160-180 °C range, around which temperature the nanoparticles decompose to give the oxides. The product selectivity and other features of the reaction are explained taking into account the various byproducts formed in the reaction. These metal peroxides also activate the CH bonds of cyclohexane. It is possible to bring down the reaction temperature by partially substituting the Zn in ZnO2 with Ni.

Reactivity of C-H bonds in cyclohexanone and 1-tert-butylperoxycyclohexanol toward the tert-butylperoxyl radical

The kinetics of oxygen uptake and the composition of the cyclohexanone oxidation products in the azobisisobutyronitrile-initiated oxidation of cyclohexanone in the presence of tert-butyl hydroperoxide have been investigated by the Howard-Ingold method. The partial rate constants of the reaction of the tert-butylperoxyl radical with the C-H bonds of cyclohexanol and 1-tert-butylperoxycyclohexanole at 333 K have been determined. The carbonyl group of cyclohexanone activates the C-H bonds in the 2- and 6-positions (α) and deactivates the C-H bonds in the 3- and 5-positions (β) compared to the C-H bonds in the 4-position (γ), whose reactivity is similar to that of the methylenic C-H bonds in cyclohexane. Evaluation of the joint effect of the hydroxyl and tert-butylperoxyl groups in 1-tert-butylperoxycyclohexanol suggests a considerable deactivation of the C-H bonds in the 2- and 6-positions (β) and, to a lesser extent, in the 3- and 5-positions (γ).

Active sites and reactive intermediates in the hydrogenolytic cleavage of C–C bonds in cyclohexane over supported iridium

Hydrogenolysis of cyclohexane has been explored over supported Ir catalysts. The kinetic data are combined with modeling results to assess the structural requirements and the nature of catalytically relevant surface intermediates for endocyclic C–C bond cleavage. The turnover frequency (TOF) for cyclohexane hydrogenolysis showed complex dependence on Ir particle size, while the selectivity to the primary ring opening product, n-hexane, decreased monotonically with decreasing Ir dispersion. The decreasing TOF as the Ir dispersion decreased from 65% to 52% originates principally from the diminishing abundance of low-coordination Ir atoms at particle surfaces. The increase of the TOF with further Ir particle growth is attributed to an increased fraction of terrace planes, or step sites, and a less unsaturated nature of the most abundant reactive intermediate. Selectivities for multiple C–C bond cleavage, yielding C<6 alkanes, varies with the relative abundance of coordinatively unsaturated Ir atoms and terrace planes. The multiple hydrogenolysis depends additionally upon H2 pressure, because single and multiple C–C bond scissions are mediated by surface intermediates with different H-deficiencies.

Metal-Free Activation of Dioxygen by Graphene/g-C3N4Nanocomposites: Functional Dyads for Selective Oxidation of Saturated Hydrocarbons

Graphene sheet/polymeric carbon nitride nanocomposite (GSCN) functions as a metal-free catalyst to activate O(2) for the selective oxidation of secondary C-H bonds of cyclohexane. By fine-tuning the weight ratio of graphene and carbon nitride components, GSCN offers good conversion and high selectivity to corresponding ketones. Besides its high stability, this catalyst also exhibits high chemoselectivity for secondary C-H bonds of various saturated alkanes and, therefore, should be useful in overcoming challenges confronted by metal-mediated catalysis.

Modeling the cis-oxo-labile binding site motif of non-heme iron oxygenases: water exchange and oxidation reactivity of a non-heme iron(IV)-oxo compound bearing a tripodal tetradentate ligand.

The spectroscopic and chemical characterization of a new synthetic non-heme iron(IV)-oxo species [Fe(IV)(O)((Me,H) Pytacn)(S)](2+) (2, (Me,H)Pytacn=1-(2'-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, S=CH(3)CN or H(2)O) is described. Complex 2 was prepared by reaction of [Fe(II)(CF(3)SO(3))(2)((Me,H) Pytacn)] (1) with peracetic acid. Complex 2 bears a tetradentate N(4) ligand that leaves two cis sites available for binding an oxo group and a second external ligand but, unlike the related iron(IV)-oxo species with tetradentate ligands, it is remarkably stable at room temperature (t(1/2)>2 h at 288 K). Its ability to exchange the oxygen atom of the oxo ligand with water has been analyzed in detail by means of kinetic studies, and a mechanism is proposed on the basis of DFT calculations. Hydrogen-atom abstraction from C-H bonds and oxygen-atom transfer to sulfides by 2 have also been studied. Despite its thermal stability, 2 proves to be a very powerful oxidant that is capable of breaking the strong C-H bond of cyclohexane (bond dissociation energy=99.3 kcal mol(-1)).

Catalytic C−H Insertions Using Iron(III) Porphyrin Complexes

Fe(III), Cu(II), and Ag(II) porphyrin complexes are active catalysts for benzylic and ring C−H insertions by carbene fragments transferred from methyl diazomalonate, 2. Temperatures above 100 °C are required, and yields greater than 70% have been achieved. C−H insertions with cyclohexane and tetrahydrofuran are catalyzed at a lower temperature of 60 °C with 60% yields when para-substituted methyl 2-phenyldiazoacetates, 15a−d, are used as carbene sources. The rate for Fe(TPP)Cl-catalyzed insertion into the C−H bond of cyclohexane was found to be first-order in the concentration of methyl 2-(p-chlorophenyl)diazoacetate, p-Cl-MPDA, indicating that formation of a carbene complex is the rate-determining step. Competition reactions for cyclohexane insertion with para-substituted methyl 2-phenyldiazoacetates correlated linearly with σ+ Hammett parameters with a ρ value of –1.11 ± 0.05 when Fe(TPP)Cl was used as a catalyst, demonstrating that electron-donating para-substituents on the phenyl group of the methyl 2...

The Effect of Catalyst Loading in Copper‐Catalyzed Cyclohexane Functionalization by Carbene Insertion

AbstractA study of the variables that affect the insertion of the :CHCO2Et group (formed from ethyl diazocetate, EDA) into the C–H bonds of cyclohexane in the presence of a TpxCu complex as the catalyst (Tpx = trispyrazolylborate ligand) has demonstrated an anomalous effect of the catalyst loading. The use of low concentrations of catalyst produces an increase in the yield of the C–H activation product ethyl cyclohexaneacetate. This effect has also been found in the case of other less elaborated catalysts such as [BpBr3Cu] or [(bipy)2Cu][I]. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)